A friend’s dad has posted a multi-part abacus math tutorial on YouTube. Okay, I know it sounds like you’re going to click the link and see Rick Astley, but no, really: It’s fascinating. I didn’t realize that the first abaci—or the abacus precursors, I guess—were probably just drawn in the dirt. Use pebbles to count. Easy.

I love that the internet provides a place for super-geeky, super-in-depth projects like this.